Biological organisms are known to provide a sensitive and reliable indicator of the presence of even relatively small proportions of toxic substances in a fluid medium. Biological organisms which have been utilized as indicators of water toxicity include organisms such as daphnia, minnows and similar complex organisms. However, tests involving these organisms normally require a substantial amount of time, i.e. on the order of several days, in order to complete the toxicity determination. Methods employing contact between microorganisms, such as bioluminescent microorganisms, and a suspect substance, have been developed for the rapid determination of toxicity. By the use of bioluminescent microorganisms, a toxicity determination can be made in less than an hour as compared to the conventional toxicity determination, which may require several days to complete.
Regardless of the type of organism employed, acute water toxicity determinations conventionally measure the effect of a toxic substance or a condition in the water on the metabolism of a living organism. A number of established test methods employing various living organisms are available. However, testing the toxicity of particulate solids such as soils, sediments, sludge and other similar particulate solid materials, has presented a problem in that such materials cannot rapidly and reliably be tested for their toxicity. Solid materials are conventionally tested by indirect methods, which utilize a solvent or acid to extract the toxic substance from the particulate solid. The solvent containing the toxic substance is then treated as necessary and tested against a living aquatic organism. It will be apparent that with any indirect method, the organism is not directly in contact with the solid sample, and this lack of direct contact may produce erroneous results. For example, with acid extraction the soil particles may be partially decomposed, and heavy metals, which are normally part of the composition of a soil, but which are not readily water soluble, may be extracted. The resultant toxicity of an acid extract may be greatly enhanced compared to that for natural leachates of the same sample. In such cases, an acid extract from a sample for which the natural leachate would be virtually non-toxic (because the slight water solubility of the metal compounds present makes them biologically unavailable to aquatic organisms) may provide a false positive indication of toxicity. In addition, indirect methods can produce erroneous results due to operator error resulting from the number of steps and chemical treatments that must be carried out before the liquid can be brought into contact with a living organism.
To avoid some of the foregoing problems, direct methods employing benthic organisms have been utilized to directly test the toxicity of a solid material. Benthic organisms are defined as those organisms which can be found at the bottom layers of ponds, lakes and the like or which normally dwell in association with a sediment or soil. Benthic organisms used in recent years for soil and sediment toxicity testing include various complex organisms such as earthworms, nematodes, oyster larvae, clam larvae and the like. Benthic organisms can be difficult to grow and harvest in sufficient quantity for reliable and reproducible test results, the tests are labor intensive (at the beginning and end) and the required test times are long. For these reasons, only one concentration of sample (100%) is normally tested using benthic organisms.
In summary, neither the direct (benthic organisms) nor the indirect methods (extracts and elutriates) are entirely satisfactory for all purposes. The choice of test methods depends largely on the question to be answered. Water elutriates cannot determine the total amounts of sparsely soluble toxic substances present in the solid phase, while organic solvent extraction and acid solution of the solids can, if and when the extracted toxicants do not precipitate when added to an aqueous test solution. Therefore, the combination of organic solvent and acid extractions gives a better indication of total toxic matter present, if and when that is the concern. However, they yield no information with regard to bioavailability of the toxicants in the original sample. Water elutriates yield the most relevant results with regard to the potential impact of leachates on aquatic organisms, while direct toxicity determinations employing benthic organisms yield results which are much more indicative of bioavailability to benthic organisms. However, as noted above, tests with benthic organisms are slow, and it is difficult to obtain reproducible results between different laboratories which must grow their own stock.